Mold closing of an injection molding machine is typically accomplished either hydraulically or mechanically via a toggle mechanism. The invention specifically applies to both closing arrangements. However, for ease in discussion, the invention will initially be described for hydraulic clamping arrangements.
A) Control systems used in Injection Molding machines to regulate mold closing. PA0 B) Control Art.
Mold closing typically occurs in a two-speed or two-step arrangement. After the mold halves open and the molded part is ejected from the mold, one of the mold members is brought rapidly into close proximity to the other mold member where its motion slows. The motion of the now slowly moving mold member is brought to a stop just as the mold members contact one another or "kiss". The plastic is then injected to complete the molding cycle.
It is appreciated that the time for mold closing comprises just one portion of the mold cycle but nevertheless is an important consideration regarding the throughput of the machine. For example, a ten second mold cycle, which is somewhat typical, could be significantly improved if the mold closing step could be improved by as little as one tenth of a second.
The mold closing mechanism uses a position sensor to determine the position of the mold halves which is typically a linear potentiometer for a hydraulic clamp and a rotary potentiometer for a toggle clamp. At the start of mold closing, the machine hydraulics move one of the mold halves at a fast rate of speed to quickly close the mold through the major portion of its travel. When the moving mold member "trips" or crosses a pre-set voltage the speed is reduced to a slower speed and when a second crossover position is reached, the mold motion stops. Now it will be appreciated that from the time the first crossover switch is actuated, some time must elapse before the flow control valve is actuated to reduce the speed. Further, once the flow control valve is actuated, the momentum of the mold carries it forward. If the machine runs only one mold, it is possible through trial and error to set the trip or crossover positions, to give a minimum mold close cycle time. In fact, improvements have been made by using an additional crossover position which is triggered prior to the time the first crossover occurs. This arming switch adjusts for the momentum of the moving mold member and provides a better control than the single crossover switch. However, the control relies on a trial and error approach for one specific part to achieve any degree of optimization. Further the control assumes the hydraulics of the machine remains essentially constant and is repeated cycle after cycle. Should there be any variation in the speed which could occur for any number of reasons between cycles or within a cycle, the momentum of the moving mold member changes affecting control, etc.
A recent development in this area has been to employ an algorithm in a programmable logic control. The algorithm determines the momentum of the moving mold member based on the speed set by the operator so that the moving mold member can be stopped at the point where it contacts the closed member. Because of the time it takes to compute the calculation by the machine's microprocessor, the speed of the moving member is set in advance based on the values set by the machine operator. The microprocessor then has sufficient time to perform the algorithm calculation (since it is performed when the command signal is set) so that an output signal is timely sent to the proportioning flow valve during mold close. So long as the speed of the moving mold member equals that set by the operator, the control is acceptable and represents an improvement over the devices discussed above. Should there be a variation between the actual speed and the set speed, the control is unresponsive. Further the control is implemented for only one stage, start-stop. There is no crossover position.
In theory, a closed loop feedback control should be ideal for this application. In practice, closed loop feedback control loops have not proven acceptable for control of mold closing for at least three reasons. The primary reason is that a closed loop having an acceptable frequency of response has not been developed. That is, a closed loop control capable of being properly tuned has not been developed, and may not be able to be developed. For example, to dissipate the stored energy or momentum of the moving mold member typically requires about 300 milliseconds. As a generally accepted rule of thumb in control theory, it takes about five times the time span of the controlled event to control the event. Thus, while the velocity of the ram can be observed and adjusted in time from increments satisfactorily within the constraints of this rule, to control the momentum of the moving mold member would take, in theory, about one and one-half seconds. This is totally unacceptable and explains why closed loop feedback control has not been used to control clamp closing. The second reason is that feedback or closed loop is traditionally known in the art as not being "time optimal". Specifically, closed loop as a control technique does not optimize or reduce to a minimum the time it takes to control the function. The last reason dictating against the use of closed loop control of mold closing is more subtle. A closed loop control requires profiling. Like the algorithm calculation noted above, the speed of the clamp is controlled throughout its travel. Injection mold machine operators typically view the control as two stage, i.e., fast-slow and slow-stop, open loop arrangements. To the extent that profiling the closing action involves change, there is some reluctance on the part of the end user to accept such controls.
In accordance with conventional programmable logic control (PLC) theory, all controls have response latency. For example, in the case of a clamp control, it can be considered that there are two processing states, namely, state A in which a mold member starts to close at a fast speed and state B in which the mold member closes at a slow speed until it contacts the other mold member whereat it stops. Upon detection of an event, E, i.e., the crossover position, the output, i.e, the proportioning control valve is set from a fast speed rate 0 to a slow speed rate 0'. In normal PLC systems, an average response latency of at least 1/2 of T.sub.A, the execution of the state A sequence instruction, will be realized in setting output 0 to state 0' and switching sequence execution to state B. Various techniques are known to reduce the response latency, T.sub.A. One method is to interrupt the scan of the sequence instructions for state A when event E is detected. Other methods involve pre-arming techniques where the analog output card is pre-programmed to respond to position E without processing a logic scan. Still another method is disclosed in U.S. Pat. No. 5,291,391 in which a specific sensor signal is sent to a "fast" processing portion of the programmable controller to independently early generate a specific output signal. Of course if the sensor information needed for event E is required in the control logic, such as that required in a molding cycle, the information is not available until the scan is complete. In general, these methods involve drastic increases in the complexity of the logic program and severe limitations in what logic processing is available when the set point or crossover position is reached. Also, these methods do not address the remaining sources of latency nor problems in controlling cycle to cycle repeatability or "jitter".
Finally, it should be noted as gathered from the material incorporated by reference herein that feed forward, state controllers, and finite impulse response filters are devices which are known per se. They have not heretofore been used in injection molding control systems because it is believed, of the complexities and peculiarities of the molding cycles performed in injection molding machines and also the processing power limitations of current controls. Further, the use of such techniques have been disclosed and discussed, in theory, with respect to single function controls employed in an external loop. They have not been conventionally used in PLC's or in combination with PLC's and external loops. Significantly, signal noise considerations have limited practical applications of state controllers or finite impulse response filters.